The permittivity ∈ of a physical medium is the relationship between the electric displacement field D and the electric field E in the material: D=∈E. The permittivity of a homogeneous material is commonly expressed relative to the permittivity of free space, ∈0, in terms of the relative permittivity, ∈r, defined by s=∈0∈r. The relative permittivity is also referred to as the “dielectric constant” of the material. The permittivity of a lossy material (such as physiological tissue) can be defined as a complex, frequency-dependent function:
            ɛ      ^        =                            ɛ          ′                -                  j          ⁢                                          ⁢                      ɛ            ″                              =                        ɛ          ′                -                  j          ⁢                      σ                          ω              ⁢                                                          ⁢                              ɛ                0                                                          ,wherein ∈′ is the real part of the permittivity, σ is the conductivity of the material, and ω is the frequency, while ∈0 is the free space permittivity.
Dielectrometers are used to measure dielectric properties, such as the permittivity, of materials. For example, U.S. Pat. No. 7,479,790 describes a capacitive plate dielectrometer, in which a small sample of a test material is inserted inside a capacitor for measurement of its dielectric properties. As another example, U.S. Pat. No. 7,868,627 describes a method for measuring dielectric characteristic of material by irradiating the material with a microwave signal and detecting the reflected signal.